Garmin 705 + Powertap + Turbo = Wrong Speed

nickd01

Whenever I use my Garmin on the Turbo with a Powertap the speed (and hence distance) the Garmin reports is completely wrong. One time it'll be 5mph, the next session 50mph. Everything else seems fine, cadence, power etc. I use it with the GPS off on the turbo. When I'm out on the road, with the GPS on, all seems fine.
The wheel size setting seems okay too.

Does anyone havbe any ideas what might be happening?

Oh - a friend borrowed my Powertap wheel and used it on his own Garmin 705 and his speed readings were fine with GPS off.

Thanks

hopper1

Not 100% sure, but do you not need a cadence/speed sensor?

DaSy

I would say that you need need the speed/cadence sensor too, it prioritises that above the PT in order of preference, so will use the GSC10 first for speed and cadence, only using the PT for power,

nickd01

Well, needing the speed / cadence sensor makes sesnse I suppose.

Do you think the Garmin is trying to infer a speed from what the PowerTap sends it? I mean, is it possible to calculate speed from Wheel Size + RPM? I'd have thought so? (My maths / physics is shoddy!)

My friend does actually have the speed / cadence sensor on his bike, but didn't have the wheel magnet on (As he was using my wheel) - do you think that's anything to do with it?

Bronzie

Set the wheel diameter in the Garmin:

Settings > Profiles & Zones > Bike Profile
Power = ticked
Wheel Size = Custom & enter wheel circumference next to it

The PowerTap has a speed sensor in the hub (counts wheel revs/sec) - entering a wheel size rather than allowing the Garmin to calculate it from GPS data will give you correct speed when using the PowerTap on the trainer

nickd01

The wheel size looked correct to me I beleive (2900 or something I seem to remember) - but I think it's "calculated" and not "custom". I'll double check this.

I've checked on the Saris site and it does say the PowerTap can calculate speed (http://www.saris.com/p-361-powertap-sl.aspx) so I've obviously got something wrong on the Garmin.

I'll check the wheel size again.

Bronzie

nickd01 wrote:

I think it's "calculated" and not "custom". I'll double check this.

That's why it's reporting incorrect speed

nickd01

Bronzie wrote:

nickd01 wrote:

I think it's "calculated" and not "custom". I'll double check this.

That's why it's reporting incorrect speed

Ah, ok. So even though the "number" is correct (I beleive) I need to force it to be custom.

It's worth a go!

Bronzie

Yes - if you let the unit "calculate" the speed from GPS, because you are not actually moving, it gets all flustered and starts making numbers up

chrisw12

Out of interest, why do you use speed on the turbo if you use a powertap?

It's just that I've total stopped looking at speed myself and was wondering if I'm missing something.

lloydy75

chrisw12 wrote:

Out of interest, why do you use speed on the turbo if you use a powertap?

It's just that I've total stopped looking at speed myself and was wondering if I'm missing something.

I like to set the turbo resistance to allow me to work at near road speeds, last season my best "10' on the turbo was slightly slower than my road time. I know with less resistance times would be faster.
Now training with PT as well, the times in interval sessions for the distance covered are not far off good road times but I only look at these when I download the data.
I know this is just a personal preference.

Bronzie

chrisw12 wrote:

Out of interest, why do you use speed on the turbo if you use a powertap?

Old habits die hard - I still like to log miles each week (even meaningless turbo miles) despite TSS having far greater relevance